This invention relates to the field of two-way radio communications and, more particularly, to an improvement in the noise blanking circuit of a receiver for such a system.
In general, blanking circuits detect noise signals added to an information-bearing carrier signal during transmission and reduce the noise perceived by the user of the receiver by, in some manner, preventing any signal from being outputted by the receiver when the received noise is excessive. In other words, a blanking circuit "clips out" tiny bits of the output when a noise pulse would otherwise be heard.
Many circuit designs have placed the blanking function in various parts of the receiver in an attempt to optimize the output signal. The problem in designing blanking circuits arises from the necessary compromise between eliminating all of the noise and, at the same time, causing no degradation in the information signal. Even relatively early patents in this field refer to the problem that occurs when a receiver's signal is blanked for too much of the time. If the signal path is interrupted for a very short period, the "hole" in the signal is filled in by stored energy and is unnoticed, but if there are too many holes, or each hole is over-extended, there will not be enough stored energy to fill in all of the holes, and noise will fill them. Partial solutions for this problem include "rate shutoff" circuits which limit the blanking action at high noise impulse rates. This is done by changing the width and amplitude of the blanking pulses when the blanking pulses exceed a given repetition rate for a predetermined length of time. Another solution is the inclusion of a level sensing circuit which decreases the sensitivity of the blanker circuit to received impulse noise in response to the received signal.
With the recent addition of surface acoustic wave (SAW) filters for delaying the received signal to allow time for the blanking circuit to blank out the noise, a new problem has arisen. A signal injected into a SAW filter does not simply exit intact at the other end at a slightly later time, but some of the energy is reflected back and forth to exit as an echo or multiple echoes. This, in effect, lengthens the received noise pulse in addition to the lengthening in the front end stages of the receiver. The more energy there is in a noise pulse, the more echoes will be apparent. This means that a high energy pulse will require a longer blanking period than a low energy pulse. As stated above, it is still desirable to use blanking pulses which are as short as possible and still blank out the noise satisfactorily.